1. Field of the Invention
The present invention relates to the area of reactive power (VAR) compensation and in particular a core form coupling transformer which is used with dynamic stabilizers to provide stabilization to AC electrical generators or turbine generators against subsynchronous resonance.
2. Description of the Prior Art
Maximizing the operational reliability and efficiency of large electrical systems frequently requires the use of reactive power (VAR) compensation. For example, industrial users commonly employ shunt capacitors, switched by conventional means, to provide the reactive power required by their load. Because use of the capacitors results in improved load power factor and load bus voltage support, significant rate savings and improvements in production process efficiency are generally realized. In a like manner, transmission engineers have applied series capacitors and switched shunt reactors and capacitors to high voltage transmission lines for purposes of increasing transmission capacity and transient stability margins, providing voltage support, and limiting steady-state and transient overvoltages.
A major problem in the application of series connected capacitor compensated transmission lines is the occurrence of subsynchronous resonance. In subsynchronous resonance, the transmission line and series capacitor exhibit series resonance at a frequency below the system frequency which is typically 60 Hz. Disturbances or faults can easily excite low frequency resonant currents. These low frequency resonant currents can affect the turbine generator sets supplying the electrical transmission line. A mechanical resonance between the generator rotor, turbine rotor and the shaft connecting the generator and turbine can be excited by these low frequency resonant currents and will continue to grow indefinitely once started. If permitted to continue, subsynchronous resonance is destructive to the machinery.
One method for controlling subsynchronous resonance is to apply a reactive load periodically to the terminals of the generator. Reactors are switched on and off by means of reverse parallel thyristor pairs. While this circuit arrangement provides compensation to the transmission line and generator, it can also cause undesirable odd harmonic currents to flow to the generator that are proportional to the size of the reactor being controlled. At times, the conduction angle of the thyristor switches is reduced to decrease the reactive current flow to the generator, a condition which accentuates the odd harmonic currents found in the stabilizer.
By arranging the thyristor controlled reactors in a delta configuration the triple odd harmonic currents, e.g. 3rd, 9th, 15th, can be substantially cancelled under balanced three phase operation. However, the other harmonic currents do not cancel. Thus, another means for providing cancellation of the odd harmonic currents other than the triple odd harmonic currents would be desirable.
In most electrical systems which utilize dynamic stabilizers, the stabilizer is ordinarily connected to the generator through a multiphase coupling transformer in order to provide a suitable operating voltage level for the thyristors and associated reactors. This transformer, usually three phase, can be of either a shell form or core form construction. Where a core form construction is used, the primary, principal secondary, auxiliary seconday windings are wound about each leg of the transformer core as shown in FIG. 1. The two auxiliary secondary windings, also known as stub windings, the principal secondary winding and the primary winding for each phase are positioned coaxially and radially adjacent, in the recited order, starting from the leg of the core.
Theoretically, the transformer that is used to interconnect the stabilizers with the generator should cause the selective cancellation of certain odd harmonic frequencies generated by the phase angle firing of the thyristors in the stabilizers. Unfortunately, the leakage reactances of the transformer tend to prevent this cancellation particularly when the reactance is a sizable percentage of the effective reactance of the dynamic stabilizers. Because of the duty cycle of the stabilizer (i.e. the short periods of time during which the stabilizer is fully on), the volt ampere rating of the transformer can be considerably less than the full-on volt ampere rating of the stabilizer. For instance, the volt amperes required may be as low as 20% of the full-on volt amperes. As a result, a situation occurs where the transformer leakage reactance adds significantly to the reactance of the stabilizer when the thyristors are conducting. Thus, it would be desirable to have a transformer that is constructed so that the effects of the leakage reactance thereof can be beneficially applied to the selective cancellation of the harmonic frequencies of currents that occur in the dynamic stabilizers.
Present designs for core form transformers cannot economically provide the proper value of leakage reactances for the windings required for the selective cancellation of subsynchronous resonance described hereinafter. As disclosed in the referenced copending related application, the leakage reactance of the secondary windings must be about twice the value of the common leakage reactance of the auxiliary secondary windings with both values being positive. Further, the ratio of the number of turns in the auxiliary secondary windings to the number of turns in the principal secondary windings should be (.sqroot.3-1)/2.+-.0.1%. The turns ratio between the primary and principal secondary windings is not specified as it is used primarily to control the voltage transformation necessary to achieve the operating voltage level required by the stabilizers. Although the turns ratios which are specified can be achieved by those skilled in the art with the arrangement shown in FIG. 1, the value of the leakage reactance of each of the principal secondary winding with this arrangement is negative. Analysis can show that for the winding arrangement shown in FIG. 1 this leakage reactance value can be made positive. However, with this arrangement the spacing required between the windings to achieve the proper values of leakage reactance for each phase would be so great that it would exceed the capacity of present commercially available transformer fabrication equipment. Thus, it would be advantageous to have a winding arrangement for a core form, forked wye transformer that would allow fabrication on existing fabrication equipment and achieve the required leakage reactance values necessary for selective harmonic cancellation.